Did a quick freeware search, and came up with a few options, for Screensaver authoring programs.
I haven't used any of these, so I can't vouch for their ease of use, or effectiveness.
These use images to make a slideshow style screensaver:Your Own ScreenSaverPhoto SaverSaver ForgeThumbsPlus Screensaver
These convert Flash into screensavers.
You would have to convert you videos to Flash, to be able to use these:FlashForgeScreenSwiftInstantStormSwiff Saver
This one looks like you can use video (AVI, MPEG, ASF, ASX, WMV, WMX)
, images (BMP, JPEG, GIF, WMF, ICO)
, formatted text documents, sound files (WAV, MIDI, MP3, WMA)
, Macromedia Flash, simple text, and other multimedia files.
This looks like it would be, your best option.MaxMedia Light Edition
If you find one that works, to your satisfaction, please, let us know about it.
You can add it to the Freeware Replacements For Common Commercial Apps
There might be others looking for such a program, and your input could prove helpful, to them.